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Philips Fluke PM6666 120MHz/1.3GHz

Since the frequency measurement on my Rigol bench multimeter is maxed at 3MHz I was planning for some time to get a dedicated frequency counter. My first choice was one of the Philips PM6669 or PM6666 counters. They can be obtained for a reasonable price and are also compact in dimensions as that is one of my biggest concerns considering the lack of desk space.

So as soon as I saw a listing for Philips PM6666 with PM9608B (1.3GHz option) for a reasonable price, I had to get it.
Seller was extremely forthcoming and with the instrument he sent a ton of documentation (datasheet, manual, service manual etc.). He also told me that he had to replace the input filter since the original one burned. Thank you CCO.

Unit itself was working and was in great condition for the instrument made in 1992/1993.
Non the less, since I plan to use it for years to come I decided to do a capacitor refresh.

Service manual contains a BOM and I used it to make an order for new capacitors.
Only capacitors with chemistry prone to aging were replaced:

Capacity Voltage Chemistry Dimensions pcs. Replacement p/n
6800uF 16V Electrolytic 18x35mm 1 NICHICON UPW1E682MHD
2200uF 16V Electrolytic 12x25mm 1 PANASONIC EEUFM1E222L
15uF 6,3V Tantalum 6,0×3,2mm 4 AVX TAJC156K016RNJ
2,2uF 6,3V Tantalum 3,2×1,6mm 7 VISHAY 293D225X9010A2TE3
1uF 16V Tantalum 3,2×1,6 mm 2 VISHAY 293D105X9020A2TE3
68uF 6,3V Solid Alu 8x8x6mm 2 NICHICON UPV1C680MGD

This is a picture of a PCB after a recap:

One thing I was not satisfied with before and after a recap was temperature stability.
Every small temperature change was immediately visible as a change in measured frequency.
That issue bothered me a lot so I was determined to find a solution. Thankfully I wasn’t the first person facing that problem.

A user “miedema” on Netherlands forum designed a complete solution and manufactured a PCB. Forum post can be found on this link:

Although the post is now four years old I contacted him and asked if he has any spare boards left to sell.
He was most kind and replied affirmatively. I quickly accepted more than fair offer for a PCB and an VCTCXO. And on top of that he added an extra VCTCXO and two 50PPM resistors used for a voltage divider together with all the documentation, parts list and datasheets. Thank you Gertjan.

With the hardest part done by Gertjan only the easy one remained. I ordered the necessary parts and proceeded with soldering.
That part was no simple feat because of the big ground surfaces that were acting quite efficient at sinking heat from my soldering station. That and the warnings about how any excess heat may damage VCTCXO or at least increase its initial drift. So I was extra careful to be quick and not to apply any unnecessary heat.
The only part I could not obtain was a voltage reference IC: REF3133AIDBZT. So I had to use its closest replacement: REF3233AIDBVT.
Since it is not direct pin to pin compatible I had to do a bodge on a PCB but it works flawlessly.

Photos of Philips Fluke 1.3GHz upgrade option PM9608B are quite scarce on internet so I added few of them together with original oscillator in hope that someone finds them useful.


  1. Franco Franco

    Ciao Lajt,
    I purchased a PM6666 as well on ebay and I was surprised at the quantity – actually the lack of quantity – of information on the web regarding this counter.
    So I enjoyed your website especially the professional photos.
    I wanted to ask you if you have additonal information on the PM6666 and maybe a contact for the gentleman to build the TCXO board. I would appreciate any help immensely.
    Thank you, Franco

  2. Lajt Lajt

    Hi Franco,

    Thank you for the comment.
    Regarding the documentation I also don’t have much, but I will send you what I do have.
    Hope it is going to be useful.

    Best regards,

  3. I like the valuable information you provide to your articles. I will bookmark your weblog and take a look at again here regularly. Larissa Paten Agueda

  4. Really appreciate you sharing this article. Really thank you! Cool. Jana Omero Schaefer

  5. Hi Lajt, greetings from Brazil! Got a 6665 and I’m creating a ocxo board and HF input board…Thanks for your info!

    • Lajt Lajt

      Thank you for your feedback.

  6. Hello Lajt,
    I purchased a PM6666 here in Denmark and would like to do the TXCO mod.
    and got inspired by your website with the the professional photos.
    I wanted to ask you if you have additonal information on the PM6666 and maybe a contact for the gentleman to build the TCXO board.
    I would appreciate any help immensely.
    Thank you, Erik Henneberg

    • Lajt Lajt

      Hi Erik,

      Thank you for your kind words.
      I will be sending all the info you requested on the e-mail.


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